There is plenty of signage - at least at the McNamara (Delta) terminal - just proceed towards "baggage claim". There is a bridge over traffic to the parking deck, which is also where all shuttle and taxi service is. The bridge is on the main terminal level, one level up from baggage claim. If they are arriving with checked bags, the down escalators to baggage claim are at the midpoint of the terminal, just beyond the security checkpoint. If they don't have checked bags, bear right past the TSA checkpoint and continue on that level to the parking deck.
Just after crossing over the road, there are two elevators and a stairway to the left. Take either down to the ground level and the taxi stand is right there.
I am sure North terminal would be similar as it is even newer. Just follow the signs.